You're right, she hasn't been in any serious fights since Sasori, but there were other times where that power would have been handy. For example, during the Pain invasion. Tsunade found Pain worthy enough of an opponent to use the Yin Seal, (if I'm remembering correctly, she uses it's power to call on Katsuyu and, through her, lend her chakra to the village), so if Sakura was also able to use that kind of power, why not assist? I would be so frustrated if I learned that she knew how to use that power all along but didn't during the invasion...
I understand I you would feel that way but let me try to explain each situation as best I can.
During the Pain attack Sakura was busy healing people and nowhere near Tsuande. Tsunade had only a split second to do all that she did to protect the village from the Pain nuke as best she could (I actually give her infinite praise for managing to do something like that so quickly). There was no time to call Sakura to aid her if Sakura truly already had the same ability.
Then there's the confrontation with Sasuke. I believe he is more than a worthy opponent to use the Yin Seal techniques on, (assuming she knew how to use them at the time), but instead of utilizing her full power, she falls back on a poisoned kunai? That just doesn't make sense to me. If she had the ability to use the techniques, why fall back on such a weak weapon?
She used the kunai because she's aware of how powerful Sasuke has become, and so came up with the best way to kill him as quickly as she could. Sasuke is a genius ninja, and so Sakura knows that eventually he would overpower her, thus her failing to kill him. That's why she used a different approach to have a better chance: deception and death by poison.
Sakura in this instance actually tried to act like a real ninja, instead of relying on power, she focused on cunning. And doing so was her best chance to kill Sasuke.
You're right, it's possible that there were other windows of time for Sakura to train that we may not have seen, but to learn the Yin Seal techniques, she needs Tsunade to train her. Post-Pain arc, the opportunities for that just aren't there. That's where I see the dilemma. After the Kage Summit Arc, Tsunade comes out of her coma, which she had been in since Naruto's arrival during the Pain fight, and is almost immediately thrown into planning a war. I just can't see how she would have found the time to train Sakura in between, all the while keeping on top of her duties as Hokage and dealing with everything else. I guess Shizune could have trained her, but then we're assuming that Shizune knows how those techniques work and can use them herself.
I don't think there's any specific confirmation that Sakura needs specifically Tsunade in order to train. Maybe Tsunade wrote down the process to master her technique in a scroll, just like it was done with the multi shadow clone jutsu, and Sakura used that scroll to train herself.
At the end what you think and what I think is all just speculation right now.
If we go off of the assumption that Tsunade did train her, but it was before the Pain arc, (let's say when Naruto was training for Rasen-Shurinken), then shouldn't Sakura have mastered those techniques by at least her confrontation with Sasuke? If not , the start of the war? But she doesn't use them at all? Not while facing Zabuza, Hakku, and the Seven Swordsmen? Not while confronting the Jyuubi, Obito, and Madara all at once? And again, if she has the ability to use the Yin Seal techniques, where is the mark on her forehead?
Why didn't she use it against Sasuke? I already explained in my post above.
Why not during the war? Because her job is to heal. There were plenty of other fighters dealing with Zabuza and the swordsmen.
Why not against Obito/Madara/Juubi? The fight is still going on. There hasn't been a need for her do so. Tsunade didn't immediately start using the strength of a hundred against Madara, it was only after she decided to go on the offensive that she did.
As for the mark in the forehead, that the one thing that can make all this theory crumble. Even I'll admit that.
I'll admit that it's possible, but I've always hoped that Kishimoto would refrain from using Ass-Pull no Jutsu on Sakura. A small time skip would be so much better than having to explain, (or ignore), all of these variables.
Izanami, never forget.